Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Replies 31 replies
  • Subscribers 36 subscribers
  • Views 3042 views
  • Users 0 members are here
Options
You may also be interested in

Protecting cables carrying high earth leakage currents located in walls and partitions against impact

TurboGen

I am investigating options for getting solar PV installed at home, thinking about how I would like the system installing ready for when I get installers round to quote.

The house was built with a service void (plasterboard boxing-in) which runs vertically from above the consumer unit in the garage, up through the first floor into the attic and this void already carries a number of other cables, within a safe zone created by the void running up the corner of the 1st floor room and with an electric shower mounted onto the void. Using this void to run the AC cables from the CU to the inverter in the attic, protecting the cables from the weather/elements seems a neater long term option than running conduit or cable externally, given the 25+ year life of the solar installation.

The cable route from the attic to CU is a vertical drop, but its not a perfect straight drop down the back wall due to a joist and some pipework obstructions around garage ceiling level, requiring some change in depth as it passes through the floor. There's plenty of space to pull in a flexible conduit, following the route of the existing T&E cables. The void is quite deep, over 100mm, so fixing a flexible conduit to the wall in the attic should keep it comfortably more than 50mm from the surface from the attic through to the 1st-floor floor/garage ceiling. But where it passes around the joist and pipes, where distance from the surface is difficult to confirm, though at this stage it is passing through the floor/ceiling.

If it was a regular household circuit with additional protection from a 30mA RCD then singles in flexible insulated conduit, running in the safe zone created by the room corner and shower, appears to be acceptable, but with most solar inverters having high earth leakage currents which require 100mA or 300mA RCDs to prevent nuisance tripping, then this would not provide the additional protection from a 30mA RCD as required under 522.6.202 / 415.1.1.

Therefore the protection against impact would depend on the >50mm distance from surface, which while this is fine for most of the drop, is less clear where it passes through the floor. Using the options from 522.6.204; a rigid conduit (BSEN 61386-21), trunking, armoured cables or mechanical protection appears impossible because they are all very rigid and the nature of the route requires a cable system with greater flexibility.

So the permitted options if I want to get the cables run through the void, appear to be:
a) Satisfy the installer the distance from the surface will be >50mm so the additional protection from a 30mA RCD is not required and 522.6.204 does not apply, so allowing any inverter and all conduit/cable options.
b) Specify an inverter that can operate with a 30mA RCD for additional protection, the cable will be in a safe zone so fulfilling requirements of 522.6.202/415.1.1, though this appears to substantially limit the choice of inverter (I've only found one so far). Again all flexible conduit/cable choices become acceptable.

Are there any other solutions using a flexible conduit or flexible cable that I have missed?

One potential solution that is not stated as a permitted option in the Regs and I'm unsure why not, would be to use a flexible metallic conduit earthed at each end via a fixed gland, with a separate CPC, protected by a 100mA or 300mA RCD. And as per (a), clipped to keep it >50mm as far as possible.

This would be at no more risk of mechanical damage than the T&E cables next to it. In the event of a nail strike or impact, any penetration is earthed by the continuous metal conduit so ensuring tripping of the RCD without relying on a current to flow through the nail/screw/person causing the strike. The RCD reduces the required trip current to only 100mA/300mA so even though the flexible conduit does not satisfy the requirements of a protective conductor, it only needs to carry a very low current to trip the RCD. But that doesn't appear to be an permitted option, any suggestions why this combination would not be permitted? 

Parents
  • 0

    +1 for SWA or indeed BS 8436 ("nail shield") cable. In smaller sizes (say up to 4mm²) SWA and BS 8436 cables are still fairly flexible - if anything I find the additional stiffness helpful for threading through voids (it tends to go where you point it rather than sagging down or coiling up) but can still bend around corners easily enough.

    Check also section 543.7 if you're having to deal with protective conductor currents >10mA- that may influence your choice of cable size as well as type.

    One potential solution that is not stated as a permitted option in the Regs and I'm unsure why not, would be to use a flexible metallic conduit earthed at each end via a fixed gland, with a separate CPC, protected by a 100mA or 300mA RCD.

    That's for consistency with regulation 543.2.3 - flexible or pliable metallic conduit is deemed unsuitable for use as a protective conductor (although it still need to be earthed as an exposed-conductive-part). It hails for problems where flexible conduit has been used in situations like final connections to motors as it tends to break over time due to movement or vibration. It doesn't seem an entirely logical result to me especially where the conduit's flexible qualities are only needed as an installation convenience rather than an in-service requirement, or indeed relying on it for shock protection for faults to the conduit itself, but that's where we are.

       - Andy.

Reply
  • 0

    +1 for SWA or indeed BS 8436 ("nail shield") cable. In smaller sizes (say up to 4mm²) SWA and BS 8436 cables are still fairly flexible - if anything I find the additional stiffness helpful for threading through voids (it tends to go where you point it rather than sagging down or coiling up) but can still bend around corners easily enough.

    Check also section 543.7 if you're having to deal with protective conductor currents >10mA- that may influence your choice of cable size as well as type.

    One potential solution that is not stated as a permitted option in the Regs and I'm unsure why not, would be to use a flexible metallic conduit earthed at each end via a fixed gland, with a separate CPC, protected by a 100mA or 300mA RCD.

    That's for consistency with regulation 543.2.3 - flexible or pliable metallic conduit is deemed unsuitable for use as a protective conductor (although it still need to be earthed as an exposed-conductive-part). It hails for problems where flexible conduit has been used in situations like final connections to motors as it tends to break over time due to movement or vibration. It doesn't seem an entirely logical result to me especially where the conduit's flexible qualities are only needed as an installation convenience rather than an in-service requirement, or indeed relying on it for shock protection for faults to the conduit itself, but that's where we are.

       - Andy.

Children
  • 0 in reply to AJJewsbury

    Re. 543.7 looks like it shouldn't be a problem, though I had to smile at 543.7.1.203 "the protective conductor being enclosed to provide additional protection against mechanical damage, for example, within a flexible conduit".

    543.2.3 - yes, the inclusion of 'subject to mechanical stress in normal service' suggests anything subject to routine mechanical stress shall not be used. As a result of the placement of the commas and "or"s it reads as a blanket exclusion of all flexible or pliable conduits", rather than "flexible or pliable conduits subject to mechanical stress in normal service".

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines